I prefer you to go with the AMD build, but go with the FX-6300 instead of FX-8320, so you can spend more on your gpu, so your gaming experience will be very good. Performance of FX-6300 with FX-8320 is not that different when it comes to gaming. Also you can use your 500GB HDD in the build too if you want to or go with 1TB for greater storage.
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: AMD FX-6300 3.5GHz 6-Core Processor (£82.49 @ Amazon UK)
Motherboard: Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard (£36.96 @ CCL Computers)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£63.00 @ Amazon UK)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£44.99 @ Novatech)
Video Card: MSI GeForce GTX 770 2GB Video Card (£269.72 @ Scan.co.uk)
Case: Aerocool DS Cube White Edition Silent Cube MicroATX Mini Tower Case (£74.99 @ CCL Computers)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply (£39.92 @ Scan.co.uk)
Total: £612.07
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-24 15:13 GMT+0000)
Also you may think that the Gigabyte radeon hd 7950 which has 3gb vram may be better than MSI Gtx 770 with 2gb vram, but the truth is GTX 770 has much better performance comparing with radeon hd 7950. You can check the comparision here:
http://gpuboss.com/gpus/MSI-GeForce-GTX-770-vs-GIGABYTE-Radeon-HD-7950